1. Field of the Invention
The present invention relates to a high frequency amplifier circuit that uses a hetero-junction bipolar transistor (HBT). In addition, the present invention relates to an integrated circuit that is used for the transmitter or receiver part of a mobile communication terminal.
2. Prior Art
In recent years, HBT's have come to be used in place of field effect transistors (FET's) in microwave monolithic integrated circuits that include power amplifiers, low noise amplifier LNA, down converter, in mobile communication terminals such as cellular phones.
The advantages of using HBT's include the following:
1) No negative power supply is required for an amplification and a gain control operation;
2) Excellent distortion properties are provided because of a high linearity in HBTs.
Contrarily, defects include the requirement of a bias circuit in order to compensate for temperature dependency and power supply dependency. How this bias circuit is designed has become an important point for gaining stable properties.
In the following, a high frequency amplifier circuit according to the prior art is described in reference to the drawings.
FIG. 6 is a circuit diagram showing a high frequency amplifier circuit that uses an emitter follower transistor type constant voltage supply bias circuit according to the prior art which is shown in Japanese Unexamined Patent Publication 2002-9558.
In FIG. 6, the emitter of a bias supplying transistor 2 for supplying a bias current to an amplifying transistor 1 is connected via a resistor 3 to the base of amplifying transistor 1 made of, for example, an HBT. This bias supplying transistor 2 provides an emitter follower configuration where the emitter is grounded via a resistor 4. In FIG. 6, the portion surrounded by a broken line is a bias circuit.
In addition, the base of bias supplying transistor 2 is connected to the base of a first temperature compensating transistor 5. Furthermore, the collector of a second temperature compensating transistor 6 and a resistor 7 are connected to the base of first temperature compensating transistor 5. The base of second temperature compensating transistor 6 is connected to the emitter of first temperature compensating transistor 5, and in addition, is grounded via resistor 8.
The emitter of second temperature compensating transistor 6 is grounded. The other end of resistor 7 is connected to a bias voltage supplying terminal 9. The collectors of bias supplying transistor 2 and first temperature compensating transistor 5 are connected to a power supply terminal 10. In addition, an input signal terminal 11 is connected to the base of amplifying transistor 1, and an output signal terminal 12 is connected to the collector. The emitter of amplifying transistor 1 is grounded.
Next, the operation of the high frequency amplifier circuit according to the prior art is described.
A regulated voltage of 2.8 V for example, is supplied to bias voltage supplying terminal 9 by means of a power supply circuit. At this time, a voltage that exceeds the turn-on voltage, approximately 1.3 V, is applied across the base and the emitter of each of bias supplying transistor 2, first temperature compensating transistor 5 and second temperature compensating transistor 6. Therefore, each of transistors 2, 5 and 6 is turned on, and consequently amplifying transistor 1 is driven.
The collector current of amplifying transistor 1 is determined by the emitter current of bias supplying transistor 2, and this emitter current is determined primarily by the resistance value of resistor 7. In addition, in order to cancel a change caused by temperature in the voltages between the bases and the emitters of amplifying transistor 1 and bias supplying transistor 2, first temperature compensating transistor 5 and second temperature compensating transistor 6 are connected in two stages.
Usually, a regulated voltage as described above is provided as the voltage that is applied to bias voltage supplying terminal 9, and therefore, the amount of change in the voltage is comparatively small. For example, the voltage may be 2.8 V+/−5%, that is to say, the amount of change, the difference between 2.66 V and 2.94 V, may be approximately 0.3 V. In the case where a regulated voltage is not supplied for some reason, or in the case where a regulated voltage does not exist within the apparatus, however, bias voltage supplying terminal 9 is directly connected to a battery which is the power supply. Therefore, the amount of change in the voltage becomes large. For example, the voltage may be 3.5 V+/−15%, that is to say, the amount of change, may be approximately 1 V.
FIG. 2 shows an example of the relationship between the voltage (hereinafter referred to as reference voltage) that is applied to bias voltage supplying terminal 9 and the collector current of amplifying transistor 1 in the high frequency amplifier circuit that uses a bias circuit according to the prior art. FIG. 2 shows the results of a case where the applied voltage is swept from 3 V to 4 V in the condition where the bias circuit is designed so as to make the collector current 10 mA when the reference voltage is 3.5 V. In FIG. 2, the broken line indicates the prior art, and the solid line indicates the below described embodiment.
As can be seen from this figure, the collector current fluctuates from 7.2 mA to 12.4 mA when the applied voltage is in a range from 3 V from 4V, and as a result, high frequency properties of the amplifier, such as power gain and distortion characteristics, also fluctuate a great deal.